The present invention is directed to ink jet printheads and, in particular, to an improved method for aligning a pre-sectioning dicing step in the fabrication of thermal ink jet dies.
Thermal ink jet printing systems use thermal energy to produce a vapor bubble in an ink filled channel to expel an ink droplet on demand. Generally, thermal ink jet printing is accomplished by the use of a printhead comprising one or more ink filled channels which communicate with a relatively small supply chamber at one end and have an opening at the opposite end. A resistor is located in each of the channels a predetermined distance upstream from the channel orifice. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. U.S. Pat. No. Re. 32,572 to Hawkins et al., whose contents are hereby incorporated by reference, discloses one embodiment of a thermal ink jet printhead and method of fabrication. A plurality of printheads are concurrently fabricated by forming a plurality of sets of heating elements with their individual addressing electrodes on one substrate surface and etching corresponding sets of grooves which may serve as ink channels with a common reservoir in the surface of a silicon wafer. The heater plate and channel plate are aligned and bonded together to form individual printheads. The individual printheads are obtained by milling away the unwanted silicon material in the etched wafer to expose the addressing electrode terminals on the substrate and then the bonded structure is diced into a plurality of separate printheads.
The bonded structure (wafer) is cut into the individual printhead modules by standard microelectronic IC wafer saws using fiducial marks on the top surface of the wafer. Besides separating the printheads, the dicing process also exposes the nozzles from which ink is ejected, on the front face of the die. In one prior art dicing procedure, the dicing blade cuts completely through the wafer and through a backing material which holds the wafer to a holding frame during the sawing process. The penalty for this complete cut is that some residue is deposited from the backing material onto the front face of the die, thereby possibly affecting the ejection of ink through the nozzles and, hence, print quality.
To circumvent this problem, an alternate dicing procedure is disclosed in U.S. Pat. No. 5,408,739 where a shallow cut is made on the back side of the wafer so that the dicing blade never "sees" any backing material. A reference cut is first made on the top surface of the wafer by aligning the fiducial marks on the top surface and then cutting off two sides of the wafer at a 90.degree. angle to each other. The back cut is then completed by aligning with the two reference cuts. U.S. Pat. No. 5,408,739 suggests that the second 90.degree. reference cuts can be eliminated by using an infrared aligner but without disclosing details of said alignment. Such alignment, in fact, has not hitherto been possible because of the lack of a mechanism for making the first back cut to meet exacting nozzle face tolerances and for lack of a suitable sensor. Vidicon cameras have proved to be too large and heavy.